Acetic acid is produced industrially by carbonylating methanol in the presence of water, a rhodium catalyst, a metal iodide, and methyl iodide. In the methanol-carbonylation process, acetaldehyde is separated and removed by using process units including a reactor for carbonylating methanol in a carbon monoxide atmosphere, an evaporator for separating the reaction mixture fed from the reactor into a volatile phase and a less-volatile phase, a splitter column for distilling the volatile phase to form a first overhead and a side-cut acetic acid stream, a decanter for condensing the first overhead to forma condensate biphasically separated into a liquid heavy phase and a liquid light phase, a distillation column for distilling the condensate fed from the decanter to form a second overhead and a bottom stream, a condenser for condensing the second overhead, and an extractor for extracting water-soluble impurities from the condensate with water forma raffinate and a water extract.
On the separation of acetaldehyde, Japanese Patent No. 3244385 (JP-3244385B, Patent Document 1) discloses a process for producing acetic acid, in which the reaction of methanol with carbon monoxide is carried out while maintaining an acetaldehyde concentration in the reaction liquid at 400 ppm or lower by removing acetaldehyde from a process liquid being circulated into a reactor. This document describes that a second overhead was subjected to water extraction with theoretical two stages to remove acetaldehyde from the process liquid and the resulting raffinate is recycled to the reactor.
According to the process, because the process stream in which acetaldehyde has been removed is recycled to the reactor, the acetaldehyde concentration in the reaction liquid can be reduced to prevent or reduce the formation of by-products.
Japanese Patent Application Laid-Open Publication No. 2007-526305 (JP-2007-526305A, Patent Document 2) discloses a process which comprises subjecting a second overhead rich in a permanganate reducing compound (PRC) with water and separating therefrom an aqueous stream, and, in order to improve aldehyde removal efficiency, recycling a first portion of the extracted raffinate to a distillation column and recycling a second portion of the raffinate to a reactor.
Japanese Patent Application Laid-Open Publication No. 2007-526310 (JP-2007-526310A, Patent Document 3) discloses twice extraction of a second overhead rich in a permanganate reducing compound (PRC) with water. The document also discloses recycling of a raffinate to a reactor and depicts recycling of a portion of the raffinate to a distillation column in drawings.
According to the processes described in Patent Documents 2 and 3, recycling dimethyl ether (DME) together with the raffinate described above to the reactor allows DME to be consumed as a raw material of acetic acid in the reactor and to prevent the inoperativeness of the process, although DME contained in a process liquid from the distillation column and DME produced in the distillation column significantly increase the pressure of the distillation column (the pressure of the column top) over a long period of time. However, according to these processes, despite precedent concentration of acetaldehyde, the high concentration of acetaldehyde is returned to the reactor, and thus the removal amount of acetaldehyde is reduced. Such an operation not only reduces the acetaldehyde removal efficiency but also increases the acetaldehyde concentration in the reaction liquid to increase the formation of acetaldehyde-derived impurities (for example, propionic acid, hexyl iodide, and crotonaldehyde). Thus, in order to improve the quality of acetic acid, it is necessary to increase an apparatus for reducing or removing substances such as propionic acid, permanganate reducing compounds (PRC's), and iodides.